The present invention relates to electromagnetic tracking devices and, more particularly, to a system and method for tracking a medical probe such as a catheter as the probe is moved through the body of a patient.
It is known to track the position and orientation of a moving object with respect to a fixed frame of reference, by equipping the moving object with a transmitter that transmits electromagnetic radiation, placing a receiver in a known and fixed position in the fixed frame of reference, and inferring the continuously changing position and orientation of the object from signals transmitted by the transmitter and received by the receiver. Equivalently, by the principle of reciprocity, the moving object is equipped with a receiver, and a transmitter is placed in a known and fixed position in the fixed frame of reference. Typically, the transmitter includes three orthogonal magnetic dipole transmitting antennas; the receiver includes three orthogonal magnetic dipole receiving sensors; and the object is close enough to the stationary apparatus (transmitter or receiver), and the frequencies of the signals are sufficiently low, that the signals are near field signals. Also typically, the system used is a closed loop system: the receiver is hardwired to, and explicitly synchronized with, the transmitter. Representative prior art patents in this field include U.S. Pat. No. 4,287,809 and U.S. Pat. No. 4,394,831, to Egli et al.; U.S. Pat. No. 4,737,794, to Jones; U.S. Pat. No. 4,742,356, to Kuipers; U.S. Pat. No. 4,849,692, to Blood; and U.S. Pat. No. 5,347,289, to Elhardt. Several of the prior art patents, notably Jones, present non-iterative algorithms for computing the position and orientation of magnetic dipole transmitters with respect to magnetic dipole receivers.
An important variant of such systems is described in U.S. Pat. No. 5,600,330, to Blood. In Blood's system, the transmitter is fixed in the fixed reference frame, and the receiver is attached to the moving object. Blood's transmitting antennas are spatially extended, and so cannot be treated as point sources. Blood also presents an algorithm which allows the orientation, but not the position, of the receiver relative to the transmitter to be calculated non-iteratively.
Systems similar to Blood's are useful for tracking a probe, such as a catheter or an endoscope, as that probe is moved through the body of a medical patient. It is particularly important in this application that the receiver be inside the probe and that the transmitter be external to the patient, because transmitting antennas of sufficient power would not fit inside the confined volume of the probe. A representative prior art system of this type is described in PCT Publication WO 96/05768, entitled “Medical Diagnosis, Treatment and Imaging Systems”, which is incorporated by reference for all purposes as if fully set forth herein. Medical applications of such systems include cismyocardial revascularization, balloon catheterization, stent emplacement, electrical mapping of the heart and the insertion of nerve stimulation electrodes into the brain.
Perhaps the most important application of this tracking is to intrabody navigation, as described by Acker in U.S. Pat. No. 5,729,129, with reference to PCT Publication No. WO 95/09562. A three-dimensional image, such as a CT or MRI image, of the patient is acquired. This image includes fiducial markers at predetermined fiducial points on the surface of the patient. Auxiliary receivers similar to the receiver of the probe are placed at the fiducial points. The signals received by the auxiliary receivers are used to register the image with respect to the transmitter frame of reference, so that an icon that represents the probe can be displayed, superposed on a slice of the image, with the correct position and orientation with respect to the image. In this way, a physician can see the position and orientation of the probe with respect to the patient's organs.
WO 96/05768 illustrates another constraint imposed on such systems by the small interior dimensions of the probe. In most prior art systems, for example, the system of Egli et al., the receiver sensors are three concentric orthogonal coils wound on a ferrite core. The coils are “concentric” in the sense that their centers coincide. Such a receiver of sufficient sensitivity would not fit inside a medical probe. Therefore, the sensor coils of WO 96/05768 are collinear: the three orthogonal coils are positioned one behind the other, with their centers on the axis of the probe, as illustrated in FIG. 3 of WO 96/05768. This reduces the accuracy of the position and orientation measurements, because instead of sensing three independent magnetic field components at the same point in space, this receiver senses three independent magnetic field components at three different, albeit closely spaced, points in space.
A further, consequent concession of the system of WO 96/05768 to the small interior dimensions of a catheter is the use of coils wound on air cores, rather than the conventional ferrite cores. The high mutual coupling of collinear coils wound on ferrite cores and measuring three independent field components at three different points in space would distort those measurements sufficiently to make those measurements fatally nonrepresentative of measurements at a single point.
Another drawback of the system of WO 96/05768 relates to the geometry of the transmitter antennas. These are three nonoverlapping flat coplanar coils, preferably arranged in a triangle. Because the strength of the field transmitted by one of these coils falls as the reciprocal cube of the distance from the coil, the receiver usually senses fields of very disparate strength, which further degrades the accuracy of the position and orientation measurements. Acker addresses this problem by automatically boosting the power supplied to transmitting coils far from the receiver. In U.S. Pat. No. 5,752,513, Acker et al. address this problem by overlapping the coplanar transmitting coils.
Acker et al. transmit time-multiplexed DC signals. This time multiplexing slows down the measurement. Frequency multiplexing, as taught in WO 96/05768, overcomes this problem, but introduces a new problem insofar as the transmitting coils are coupled by mutual inductance at non-zero transmission frequency, so that the transmitted field geometry is not the simple geometry associated with a single coil, but the more complex geometry associated with several coupled coils. This complicates and slows down the calculation of the position and orientation of the receiver relative to the transmitter coils. PCT Publication WO 97/36143, entitled “Mutual Induction Correction”, addresses this problem by generating, at each transmitter coil, counter-fields that cancel the fields generated by the other transmitter coils.
A further source of slowness in calculating the position and orientation of the receiver is the iterative nature of the calculation required for a spatially extended transmitter. As noted above, Blood calculates the position of the receiver iteratively. Even in the DC case, Acker et al. calculate both the position and the orientation of the receiver iteratively.
There is thus a widely recognized need for, and it would be highly advantageous to have, a faster and more accurate method for tracking a medical probe inside the body of a patient.